1. Field of the Invention
This invention relates to a semiconductor laser, and more particularly to a semiconductor laser having a rugged grating which extends in the longitudinal direction of a laser stripe including an optical waveguide and an active layer.
2. Description of the Related Art
A semiconductor laser which oscillates in a single-longitudinal mode is known as a light source for a mass optical communication. Conventionally, a distributed feedback (DFB) semiconductor leaser has been used, which has an optical waveguide including a grating for distributing and feeding back light to the top or bottom portion of an active layer. A type of the distributed feedback laser has laser stripe constituted by an optical waveguide layer including a grating, an active layer, and a cladding layer. Whether the distributed feedback laser oscillates in a single-longitudinal mode or not depends on the phase of the grating at the end face of the laser stripe. To increase the probability of single-longitudinal mode oscillation, it is necessary to control the shape of the grating.
Usually, a grating is formed by wet-etching. In a semiconductor laser for optical communication, which generates a beam of a wavelength of 1.3 .mu.m or 1.55 .mu.m, convexities of the one-dimensional grating are arranged at intervals of 2000 to 2400.ANG., and the height of each convexity is, at most, several hundreds of .ANG.. Hence, it is very difficult to control the shape of the rating by wet etching.
Further, to increase the probability of single-longitudinal mode oscillation, there has been proposed a structure in which the phase of the grating is shifted at the middle of the laser stripe. However, since a complicated process is required to manufacture this structure, it is also difficult to control the shape of the grating.